Lamellar crucible for induction melting titanium

ABSTRACT

A lamellar crucible for induction melting titanium has an outer graphite shell closed at one end to form inner and outer surfaces. A first layer generally of the class of refractory metals or of beryllium oxide covers the inner surface of the shell to limit carbon contamination. A second layer is preferably oxygen deficient yttrium oxide and may also be generally of the class of rare earth metal oxides in an oxygen deficient condition. Also, the second layer may be yttrium oxide or the rare earth metal oxides. The second layer covers the first layer to limit reactivity with the molten titanium.

Zanis et al.

ted ES atent 1 [451 May 22, 1973.

[54] LAMELLAR CRUCIBLE FOR 3,417,808 12/1968 Rosenberg ..l64/68INDUCTION MELTING TITANIUM P E G 1d A. D t {75] Inventors: Charles A.Zanis, Bowie; Richard L. :3:3;, g g: and Hodges Helierich, Crownsville,both of Md. l

[73] Assignee: The United States of America as [57] ABSTRACT representedy the Secretary of the A lamellar crucible for induction meltingtitanium has Navy, washmgwn, an outer graphite shell closed at one endto form inner [22] Filed: 8, 1972 and outer surfaces. A first layergenerally of the class of refractory metals or of beryllium oxide coversthe [21] Appl. No.: 224,537 inner surface of the shell to limit carboncontamination. A second layer is preferably oxygen deficient yttriumoxide and may also be generally of the class of [52] US. Cl. ..266/39,263/48, 266/43 rare earth metal oxides in an oxygen deficient condi [51]Illt. Cl ..F27b 14/10 tion. Also, the Second layer may be yttrium oxideor [58] Field of Search ..263/48; 266/39, 43; the rare earth metalOxides The second layer covers 164/52, 68 the first layer to limitreactivity with the molten titanium. [56] References Cited 4 Claims, 2Drawing Figures UNITED STATES PATENTS 2,874,953 2/1959 Rogers et al..266/43 MOLTEN 4 r TITANIUM LAMELLAR CRUCIBLE FOR INDUCTION MELTINGTITANIUM The invention described herein may be manufactured and used byor for the Government of the United States of America for Governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates generally tometallurgy and specifically to rare and refractory metals.

DESCRIPTION OF THE PRIOR ART Since molten titanium dissolves almost allmaterials, the major problem encountered during melting is containingthe liquid metal without picking up contaminants. Several processes havebeen developed which utilize a solid titanium film or skull on theinside of a liquid-cooled copper crucible. The titanium is melted eitherby striking an arc between a titanium electrode and the skull, or bymelting off a titanium electrode with an energy source such as anelectron beam. This skull melting technique has been the primary methodused by industry to produce cast titanium components.

Skull melting has several disadvantages which affect the soundness andhomogeneity of the resulting castings. The copper crucible maintains asteep thermal gradient across the melt which results in compositionalvariations within the crucible. Superheating of the melt is limited.Consequently, the molten metal does not possess sufficient fluidity tofill the mold properly and yield a sound casting. The melting processdoes not impart a sufficient stirring or mixing action in the melt, andcompositional heterogeneity may be encountered.

Induction melting is one alternative to skull melting which would lessenthe difficulties heretofore known. However, one obstacle to successfulinduction melting of titanium is the lack of a suitable crucible.

Achievement of a suitable crucible for melting titanium requires boththe selection of materials which are not attacked by molten titanium andthe development of crucible fabrication techniques which provide a highdensity interface with the molten metal. A high melting point, favorableelectrical properties and high resistance to thermal shock of graphitefavor its use as a crucible material. However, the detrimental effectsof carbon contamination on the properties of titanium require thatcoatings or layers be utilized to isolate the graphite from thetitanium.

High purity yttrium oxide, Y O has been found to be substantiallyresistant to dissolution by molten titanium. However, due to the cost ofyttrium, a solid yttrium oxide crucible is not practical. Also, such acrucible has relatively poor thermal shock resistance and thus is not asfavorable for induction melting as graphite. Yttrium oxide coateddirectly on graphite is relatively unfavorable also since the yttriumoxide does not adhere well to graphite. Furthermore, yttrium oxide indirect contact with graphite will break down at certain temperaturescausing oxygen to bubble out through the coating and the melt thuscontaminating the titanium.

SUMMARY OF THE INVENTION The present invention provides a suitablecrucible for induction melting titanium. This has been accomplished byproviding a lamellar crucible comprising a graphite outer shell. A firstlayer or coating, which may be generally of the class of refractorymetals or beryllium oxide, abuts the inner surface of the shell to limitcarbon contamination of a second layer and of the molten titanium. Thesecond layer abuts the first layer and is preferably oxygen dificientyttrium oxide and may be generally of the class of rare earth metaloxides in an oxygen deficient condition to limit reactivity with themolten titanium. Also, the second layer may be of yttrium oxide or therare earth metal oxides. The outer shell is preferably a graphitecylinder of high purity having a substantially low ash content to aid inreducing contaminants which may affect the molten titanium. Also, a lowash content is advantageous since a high ash content would result in theash burning off rapidly thus causing the eventual breakdown of thegraphite. As a result, a low ash content increases the thermal stabilityof the graphite shell. Additionally, a substantially high density carbonresults in more efficient and more even heating of the shell.

OBJECTS OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a crucible suitable for induction melting titanium.

It is another object of this invention to provide such a crucible tosubstantially limit contamination of the molten titanium.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawing wherein:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view in cross-sectionalelevation illustrating the novel lamellar crucible; and

FIG. 2 is an enlarged partial cross-sectional view illustrating thethree layers of the crucible of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. l, thelamellar crucible is generally designated 10 and comprises an outershell 12, a first layer 14, and a second layer 16, see also FIG. 2.

Shell 12 is preferably formed of machined graphite due to its highmelting point, high resistance to thermal shock and favorable electricalproperties. However, other suitable materials having similar propertiesmay be used in place of graphite. Shell 12 is generally formed as acylinder having one end 18 closed. Thus, graphite shell 12 has inner andouter surfaces 20 and 22 respectively.

Preferably, graphite shell 12 has an ash content of less than 0.04percent. This is important since the percentage of ash in the graphitedetermines the carbon purity of the graphite. For example, the ash burnsoff at a lower temperature than the carbon, therefore, if the ashcontent is high then more ash burns off upon heating of the crucible. Asa result, the crucible will eventually suffer a structural breakdown.Also, high ash content affects the thermal stability of the cruciblesince the gradual burning off of ash affects the crucible qualitatively.Therefore, a relatively highly pure carbon having a low ash content islonger lasting and has substantially greater thermal stability thancrucibles having a higher ash content.

If titanium were melted in the graphite crucible, the carbon in thegraphite would contaminate the molten titanium. Although a high puritygraphite can limit carbon contamination, it cannot eliminate italtogether.

it is also preferred that such a crucible be of substantially highdensity such as, for example, a density of at least 1.5 grams per cubiccentimeter. A crucible of such a density noticeably aids in promotingeven and efficient heating and a shorter heating cycle.

As has been stated, carbon contamination cannot be eliminated,therefore, in order to utilize the preferred qualities of a graphitecrucible, there is a need to lessen the effects of carbon contaminationin the molten titanium. In order to accomplish this, a barrier orcoating of suitable material must be used to cover the inner surface 26of crucible 12 as shown in FIG. 1. A first layer 14 of material selectedfrom the group consisting of tungsten, tantalum, molybdenum, niobium,hafnium, generally known as refractory metals, and also berylliurn oxideis provided to cover inner surface of shell 12. The tungsten coating ispreferred although each of these materials produce substantiallyequivalent results, i.e., they limit carbon contamination of both themolten titanium and of a second layer which is directly in contact withthe melt.

The second layer 16 is provided to cover the first layer 14 so as to bein direct contact with the molten titanium, see FIG. 1. Layer i6 is ofmaterial selected from the group consisting of oxygen deficient yttriumoxide, yttrium oxide, oxygen deficient rare earth metal oxides and rareearth metal oxides. The oxygen deficient yttrium oxide is the preferredcoating although each of these materials produce substantiallyequivalent results, i.e., they are substantially resistant todissolution by molten titanium. Also, the oxide coating or secondcoating 16 is out of direct contact with crucible 12 so as to noticeablylimit carbon contamination. By oxygen deficient is meant a materialwhich has a greater affinity for oxygen than the same material which isnot oxygen deficient. Thus, the second layer of material is preferablyoxygen deficient and has a greater affinity for oxygen than the moltentitanium so as to limit the oxygen contamination of the melt. The rareearth metal oxides and yttrium oxide in their relatively pure forms maybe caused to become oxygen deficient by heating the materials undernon-oxidizing conditions such as in an inert or a hydrogen atmosphere orin a vacuum. These conditions will cause the pure oxide to give upoxygen, thus the oxygen deficiency results.

Shell 12 is preferably formed by machining. Firs layer 14 is preferablyapplied by the electron beam technique. However, layer 14 may be appliedby plasma spraying, by other standard powdered metallurgical techniquesfor coating materials or by an insert. Second layer 16 is preferablyplasma sprayed to layer 14, however, layer 16 may be accomplished byslip casting or by an insert of pre-sintered yttrium oxide.

Thus it may be seen from the foregoing that a substantially satisfactorylarnellar crucible may be provided for induction melting titanium so asto limit carbon contamination of the melt while utilizing the beneficialand favorable qualities of graphite.

Obviously, many modifications and variations of the present inventionare possible in light of the teachings. It is therefore to teachings. Itis therefore to be understood that within the of the appended claims theinvention may be practiced otherwise than as specifically described.

What is claimed is:

l. A lamellar crucible for induction melting titanium comprising:

an outer shell closed at one end, said shell having inner and outersurfaces;

a first layer of material selected from the group con-' a density of atleast 1.5 grams per cubic centimeter.

2. The crucible of claim 1, wherein the outer shell comprises a graphitecylinder.
 3. The crucible of claim 2 wherein the graphite has an ashcontent of less than 0.04 percent.
 4. The crucible of claim 3 whereinthe graphite has a density of at least 1.5 grams per cubIc centimeter.